This invention relates to a field effect semiconductor device, more particularly a field effect semiconductor device comprising a semiinsulator layer composed of such a relatively high resistivity semi-conductor material, such as a P.sup.-, N.sup.-, or I type material, an N type active layer adjacent to the semiinsulator layer made of the same material as the semiconductor and acting as a channel, a cathode electrode and an anode electrode which are disposed on the surface of the active layer, and means for varying the electron current flowing through the active layer by impressing voltage between the cathode and anode electrodes so as to vary the current through the active layer, wherein the cathode electrode is in ohmic contact with the active layer.
Among the semiconductor devices of the type referred to above are three terminal devices such as a junction type, MIS (metal insulated semiconductor) type and MES (metal semiconductor) type, and two terminal devices such as a planar type Gunn diode. The problems discussed hereinbelow occur in any one of the types, but for the sake of illustration a planar type Gunn diode will be described as a typical example.
As disclosed in the technical publication authored by Masakazu Shoji entitled "Improvement of Reliability of Gunn Diodes", Proceedings of the IEEE, Feb. 1969 pages 250 and 251, in a planar type Gunn diode, such III-V group compound semiconductors as gallium-arsenide and indium-phosphide are used as a channel active layer and the cathode and anode electrodes are connected to the channel active layer with an ohmic contact, the cathode and anode electrodes being located at a definite spacing on the channel active layer. This Gunn diode generally has a characteristic that upon application of a bias voltage across the anode and cathode electrodes, the current increases in proportion to the applied voltage while the field is weak but as the field in the active layer increases beyond the threshold field for the Gunn effect of about 3.5 KV/cm, the current saturates with instantaneous current oscillation.
The mechanism of the above oscillation is explained as follows: one cycle of the oscillation corresponds to (i) nucleation of the Gunn domain around the cathode area, (ii) its transit through the channel and (iii) its vanishing around the anode area. Repetition of the above cycle is the oscillatory change of the output current as RF output current.
Since the impedance of the Gunn domain is inherently high, the current through the channel decreases when the Gunn domain nucleates and this lower level of the current (valley current level) persists until the Gunn domain diminishes; i.e. the current increases to the previous saturated level again at the instance of the Gunn domain vanishing around the anode area. If a Gunn diode is biased just below the threshold (just below the saturated current level) for Gunn effect, an input of a trigger pulse, which can increase the field in the active layer beyond the threshold field for the Gunn effect, will produce the Gunn domain. The above trigger mode operation of a Gunn diode can perform a kind of pulse regeneration useful in digital signal processing devices.
If a Gunn diode is biased so high that the field in the active channel is above the threshold field for the Gunn effect, the diode works as a continuous wave oscillator. In the above description, trigger mode operation and continuous oscillation mode operation are explained for a Gunn diode. For these two modes of operation, the RF output current is given by difference of current levels between the above mentioned saturated current level and the valley current level. The ratio of the above difference between the both current levels to the saturated current level itself is called the current drop ratio.
In reality, a conventional planar Gunn diode produces smaller RF output compared to the theoretical value, moreover, its frequency spectra contains a considerable fraction of undesirable noise. In addition the bias voltage necessary to produce the Gunn effect is larger than the theoretically expected value and this larger required bias voltage causes a burn out phenomenon around the anode electrode, especially between the anode electrode and the immediately underlying active layer.
It should be noted that the above mentioned characteristic deviations of the conventional planar type Gunn diode from the theoretically expected characteristics become relatively more prominent under DC bias operation when compared to the case of pulse bias operation.
For this reason, the expected usefulness of a Gunn diode as a frequency element operating at high frequencies higher than microwave frequencies has been difficult to realize practically.
Various analyses have been made to solve these problems and the following two phenomena were noted.
The first phenomenon is that upon application of a voltage across the anode and cathode electrodes, a stable Gunn domain stays around the anode area, where electrons flow from the N type active channel layer to the N.sup.+ area under the anode electrode in which electrons concentrate. The generation of such a phenomenon was reported by Shinya Hasuo et al in their technical publication entitled 'Influence of Carrier Diffusion on an Anode Trapped Domain Formation in a Transferred Electron Device", I.E.E.E Transaction Electron Devices, Ed-23, 1976, pages 1063-1069. The occurrence of this phenomenon results in an increase in the internal resistance of the device.
The other phenomenon is that the space charge layer of a NN.sup.- or NP.sup.- or a NI transition region formed between a semiinsulator substrate and an active layer thereon when a normal bias voltage is impressed across the anode and cathode electrodes tends to increase. More particularly, in the vicinity of the anode electrode where electric flux crosses the interface from the active layer to the semiinsulating substrate, a positively charged space charge layer consisting of donors alone is formed on the side of the active layer of the transition region, whereas on the side of the semiinsulator substrate a negatively charged space charge layer is formed. The cause of generation of negative space charge in the side of semiinsulator substrate facing the positive space charge in the active layer side is natural in the case of an NP.sup.- combination for the active layer-semiinsulator structure, because the bias between the anode electrode and cathode electrode results in the application of a reverse bias onto the NP.sup.- junction in the vicinity of the anode electrode where electric flux invades into the semiinsulating substrate. But if the semiinsulating substrate is N.sup.- or I type, the cause of generation of negative space charge in the semiinsulating substrate is not necessarily straightforward. The tendency of negative charging for an N.sup.- or I type substrate is explained by the existence of electron trapping centers contained in real crystal and this phenomenon of negative charging in substrates has been reported by T. Itoh et al in their technical publication entitled "Interface Effects on Drain Current Instabilities in GaAs MESFETs with Buffer Layer", Digest of Technical Papers, The 11th Conference on Solid State Devices, Tokyo, 1979, pages 85 and 86. For this reason, a certain number of electrons which invade into the semiinsulator substrate tend to be trapped by the space charge layer formed in the semiinsulator substrate. The above growth of negative charge in the semiinsulating substrate side induces an enhanced growth of positive space charge in the facing active layer side. For this reason, the internal resistance of the device is increased. The latter phenomenon narrows the flowing path of the electrons in the active layer near and below the anode electrode thereby enhancing an increase in the internal resistance of the active layer. Due to concurrent occurrence of these two phenomena, the entire active layer near the anode electrode becomes a space charge layer, so that even when the voltage impressed across the anode and cathode electrodes is increased the increment thereof merely contributes to the growth of the space charge layer, with the result that it becomes impossible to prevent a rapid increase of the field in the active layer and thereby to realize a fast travelling state of sufficiently accelerated electrons. For this reason, the saturation value of the current available at the output of the device is undesirably limited. At the same time, the growth of the space charge layer increases the field near the anode electrode thus causing ultimate rupture of the device.